Many affective disorders are caused or exacerbated by exposure to severe or repeated stressors. Despite the important role of stressor exposure in modulating emotion, the mechanisms by which central emotional circuits are altered by stress are still unknown. Substantial evidence has suggested that the bed nucleus of the strain terminalis (BNST) mediates anxiety-like behavior in humans and animals, and it is likely that altered BNST function underlies anxiety disorders. In addition to coordinating anxiety-like behavioral responses, the BNST also organizes stress responding via projections to the paraventricular nucleus of the hypothalamus (PVN). We now have substantial data suggested that pituitary adenylate cyclase activating polypeptide (PACAP) activation and release in the BNST mediates many of the behavioral effects of repeated stress, since BNST PACAP antagonism blocks the behavioral consequences of repeated stress, BNST PACAP infusion mimics many of these behavioral consequences, and repeated stress or glucocorticoid treatment substantially elevates BNST PACAP in rats. Moreover, we have also recently shown that circulating PACAP levels and a unique single nucleotide polymorphism in the PAC1 receptor predict PTSD symptoms and diagnosis in women, suggesting that PACAP systems are indeed altered in anxiety disorders. Hence, the Aims of this proposal were designed to investigate mechanisms by which PACAP may modulate BNST activity, whether these mechanisms differ in male and female rats, and whether these differences depend on estrogen. Aim 1 will characterize the anatomy of different BNST subregions that potentially underlie the effects of BNST PACAP activation by stress. Aim 2 will investigate mechanisms by which PACAP can enhance the excitability of BNST neurons. Aim 3 will investigate the role of PACA in stress-induce enhancement in BNST neuroplasticity. Aim 4 will investigate sex difference in BNST PACAP circuitry and the dependence on estrogen. These studies will investigate mechanisms by which PACAP, a peptide only recently implicated in stress-related psychopathology, might mediate emotional behavior, by enhancing excitability and plasticity in a brain region critical for anxiety-like behavior, the BNST.